1. Field of the Invention
The present invention relates to power supplies and more precisely to the operation of power supplies which feature a maximum on the curve of the power supplied as a function of the voltage at the terminals of the supply.
2. Description of the Prior Art
For the above kind of supply, the power supplied is at a maximum when the voltage has a given value. For optimum operation of the power supply—to draw the maximum power therefrom—it is beneficial for the voltage at the terminals of the supply to be as closely equal to the aforementioned given value as possible.
The solar generators used for satellites constitute one example of the above kind of power supply. FIG. 1 is a graph of the current and the power as a function of the voltage at the terminals of the generator for a generator formed of a series connection of 102 back surface reflector (BSR) Si cells; cells of the above kind are available in the aerospace industry. The current in amperes supplied by the solar generator and the power delivered by the generator in watts are plotted on the ordinate axis; the voltage in volts at the terminals of the generator is plotted on the abscissa axis. The curves 1 and 2 in FIG. 1 correspond to operation at a temperature of +100° C. and the curves 3 and 4 correspond to operation at a temperature of −100° C. The curve 2 in FIG. 1 is a graph of the current as a function of voltage and shows that the current supplied by the cells falls when the voltage exceeds a value of the order of 35 V, which is caused by saturation of the cells; the curve 4 is similar, except that the saturation voltage is of the order of 75 V. The curve 1 in FIG. 1 is a graph of the power supplied by the solar generator and shows that the power supplied has a maximum value in the example of the order of 100 W which is achieved for a value V0 of the voltage that is of the order of 38 V. The curve 3 is similar to the curve 2, with maximum power and voltage V0 values of the order of 200 W and 70 V, respectively. These curves constitute only one particular example of a generator in which the graph of the power supplied as a function of the output voltage features a maximum.
When using the above kind of solar generator, or more generally the above kind of power supply, it is beneficial for the voltage at the terminals of the supply to be as close as possible to the value V0 of the voltage at which the supply delivers maximum power. This problem is particularly acute in the case of solar generators used on satellites. For these solar generators, the voltage V0 at which the generator supplies the maximum power varies as a function of the temperature of the generator, as shown in FIG. 1, and the voltage V0 also varies as a function of:                the intensity of the solar radiation to which the generator is exposed, and        aging of the generator.        
The temperature of a satellite typically varies within a range from −100° C. to +100° C. in the case of a satellite in low Earth orbit, for example. For a Mercury orbit, the temperature variation is even greater, and the temperature can vary over a range from −150° C. to +250° C. The intensity of the solar radiation can vary as a function of the distance from the Sun; for a mission from the Earth to Mars, the intensity of the solar radiation can vary in a ratio from 3 to 1. Aging of the generator short circuits some cells. Overall, the voltage V0 can typically vary in a ratio from 1 to 2, for example from 40 V to 80 V.
It has therefore been proposed, in order to extract maximum power from them, to operate solar generators in such a way as to have the voltage at the terminals of the generator close to the voltage V0. The techniques for achieving this are known generically as maximum power point tracking.
W. Denzinger, Electrical Power Subsystem of Globalstar, Proceedings of the European Space Power Conference, Poitiers, France, 4-8 Sept. 1995, describes the power subsystem of the Globalstar satellites. The maximum power point is determined by considering it to have been reached when the dynamic impedance of the generator is equal to the static impedance, in other words when:
 V/I=dV/dI
that is to say when:dI/I=dV/VStrictly speaking, VI=max implies VdI+IdV=0 and thus V/I=−dV/dI. Denzinger forgets the − sign.
The above document describes a circuit using a current sensor, a voltage sensor, two sampling circuits, two comparators, a bistable and an integrator.
Kevin Kyeong-II Choi and Alphonse Barnaba, Application of the maximum power point tracking (MPPT) to the on-board adaptative power supply subsystem, CNES technical memorandum No. 138, Jul. 1998, describes an electrical power supply subsystem for low-power satellites. For maximum power point tracking, this subsystem uses a microcontroller associating digital multiplication of the current by the intensity and an algorithm for tracking the power on the basis of the calculated values.
These solutions are complex to implement. They lead to centralizing control of maximum power point tracking of the various solar generators, and this centralization affects the reliability of the electrical power supply subsystem and is incompatible with maximum power points at different voltages in different sections of the solar generator. Furthermore, these solutions use the direct components of the currents and/or voltages, which are not characteristic of maximum power point tracking.
This problem, explained here with reference to satellite solar generators, arises more generally for any power supply whose graph of the power supplied as a function of voltage features a maximum.
There is therefore a requirement for a solution for operating a power supply so that the curve of the power supplied as a function of the voltage at the terminals of the supply features a maximum. Such a solution should, using means that are as simple and as rugged as possible, ensure that the voltage at the terminals of the power supply is as far as possible as close as possible to the voltage at which the maximum power is supplied.